Maritrema arenaria is a digenean trematode parasite, whose eggs are ingested by the Rough Periwinkle, Littorina saxatilis. This snail is native to both sides of the North Atlantic, and was probably introduced to San Francisco Bay, with seaweed used to pack baitworms. The cercaria develop in the periwinkle, and are released and infect barnacles, primarily the Rock Barnacle Semibalanus balanoides, where they develop into metacercariae, which develop into cysts The barnacles are eaten by shorebirds, predominantly the Ruddy Turnstone (Arenaria interpres), where they develop into adult flukes, whose eggs are deposited in the intertidal zone.